During the late 1970s, Radosh gained widespread notoriety for arguing, based on declassified FBI documents and interviews with their friends, that Julius and Ethel Rosenberg were guilty of spying for the KGB. According to his close friend David Horowitz, Radosh's decision to publish his findings led to his social ostracism.

His most recent book, co-authored with his wife Allis Radosh, is A Safe Haven: Harry S. Truman and the Founding of Israel, published by HarperCollins in 2009.[2] They are currently writing a book about the Presidency of Warren G. Harding, to be published by Simon & Schuster.

Radosh was born in New York City. His parents, Reuben Radosh and Ida Kretschman, were Jewish immigrants from Eastern Europe. He has stated that his earliest memory is of being taken to a May Day parade in New York's Union Square.[3]

In 1959, he arrived at the University of Iowa intending to work towards his Master's Degree. Despite being raised as a red diaper baby by fellow travelers, Radosh was shocked by revelations of Stalin's crimes which began to be released during the Khrushchev thaw. Although he had been a leader of Madison's Labor Youth League, he eventually broke with the Soviet-backed Communist Party USA and became a founding father of the American New Left.[9] Radosh's fondness for the writings of Isaac Deutscher enraged the Madison Communist Party cell. Their attempts to bring him back into the party line was a major part of Radosh's break with Communism.[10] In 1963, he returned to New York City with his wife and children.

When Norman Thomas died in 1967, I wrote what may have been the only published negative assessment of his life. Most obituaries heralded Thomas as the nation's conscience, a man of principle who had turned out to be right about a great deal. Of course, Thomas was against the war in Vietnam; he had made a famous speech in which he said he came not to burn the American flag but to cleanse it. But for radicals like myself, that proved that he was a sellout. His opposition to the war was so tame, I argued, that he actually helped the American ruling class. I claimed that Thomas' opposition to LBJ's bombing campaign was only a "tactical" difference with the President. Thomas' chief sin, in my view, was to have written that he did not, "regard Vietcong terrorism as virtuous". He was guilty of attacking the heroic Vietnamese people, instead of the United States, which was the enemy of the world's people. My final judgment was that Thomas had "accepted the Cold War, its ideology and ethics and had decided to enlist in fighting its battles" on the wrong—the anti-communist—side.[11]

In his book Prophets on the Right, completed in 1974, Radosh referred to himself as both "an advocate of a socialist solution to America's domestic crisis" and "a radical historian".[13] The book profiles a several historical conservative or far-right isolationists, "critics of American globalism", men who were "outside the consensus, or the mainstream... [and] regarded as subversive of the existing order"; Radosh's stated aim in writing the book was to "move us... to think carefully about alternative possibilities" to "our current predicament"—a clear reference to the still-ongoing Vietnam War.[14]

In the 1983 book The Rosenberg File, he and co-author Joyce Milton conclude that Julius Rosenberg was guilty of espionage and that Ethel was aware of his activities. A second edition, published by Yale University Press, in 1997 incorporates newly obtained evidence from the former Soviet Union. Radosh and Milton also condemned prosecutorial misconduct in the case. As a result of their 1983 book and revelations in the Vassiliev papers as well as the Venona decrypts, a consensus has emerged that rather than a frameup by the US government, both Rosenbergs were Soviet agents, and Julius Rosenberg was an agent who put together an active network that stole significant military secrets for the Soviet Union. Co-defendant Morton Sobell's 2008 interview with Sam Roberts of The New York Times had him admit his own guilt and that of Julius Rosenberg, after years of proclaiming his innocence, supporting the thesis of The Rosenberg File. A year later, Radosh and Steven Usdin held an interview with Sobell; writing in The Weekly Standard, Sobell outlined the dimensions of the material he passed to the Soviets as part of the Rosenberg network.

His memoirs, published in 2001, are titled Commies: A Journey Through the Old Left, the New Left, and the Leftover Left. The memoir discusses the various reasons for his disillusionment with the Left and utopian Marxist solutions, including his mid-1970s trip to Cuba, his experiences in Central America in the 1980s, and how he was read out of the Left because he dared to tell the truth about the espionage of the Rosenbergs.

Radosh is currently an adjunct fellow at the Hudson Institute in Washington, D.C.,[16] and professor of history emeritus at the City University of New York,[17] where he was a faculty member at both Queensborough Community College and the Graduate Faculty in History at CUNY.

Radosh married Alice Schweig on the summer of 1959. He recalls, "Our wedding was on Labor Day weekend, and after the ceremony we drove into New York to spend one night in town. We celebrated our wedding by watching the annual proletarian Labor Day parade that still marched through downtown New York."[18] They separated in 1969 and later divorced.[19]

In October 1975, Radosh married Allis Rosenberg,[20] who has a PhD in American History and has co-authored two books with him. The couple reside in Silver Spring, Maryland.[21]

Radosh reviewed Diana West's American Betrayal in FrontPage Magazine. He criticized her limited knowledge of the scholarly literature and called her thesis—that infiltration of the U.S. government by Stalinist agents and fellow-travelers significantly altered Allied policies during World War II so as to favor of the Soviet Union—a "yellow journalism conspiracy theory".[22]Michael J. Totten also praised Radosh's "masterful takedown".[23]John Earl Haynes and Harvey Klehr, scholars of Soviet espionage, came to the defense of Radosh in an article rejecting the crucial contention that Roosevelt's right-hand man, Harry Hopkins, was a Soviet spy.[24]Vladimir Bukovsky, a Soviet dissident, describes Radosh's review as dishonest and full of distortions.[citation needed] Numerous[weasel words] conservative authors[who?] and commentators also strongly criticized both Radosh and the small group of writers who rushed to his defense, and the event turned into a seminal controversy within conservatism in recent years.[citation needed] West published a follow-up book focusing on the attack on her by Radosh and others; Radosh has acknowledged that some of his assertions in his initial critique were not accurate.[citation needed] The journal The New Criterion had a full-fledged dialogue about the issues that arose due to his critique of West.